The invention relates to a process for regenerating an aqueous solution containing metal ions and sulfuric acid, especially a solution containing zinc ions, nickel ions, iron ions and/or copper ions, in an electrolytic cell, wherein the metal ions are precipitated on the surface of the cathode and oxygen and protons are formed at the anode by hydrolysis, and regenerated solution can be returned to a preceding etching process or extraction process, as well as to an apparatus.
A disclosure is made in the textbook, "Praktische Galvanotechnik," published by Leuze Verlag of Saulgau/Wurttemberg, 1970, pages 537-538, of precipitating zinc out of sulfate electrolytes. Such sulfate electrolytes form in the conversion of zinc chloride solutions into zinc sulfate solutions by ion exchange methods, in which this preliminary step is intended to avoid any electrolytic treatment of chloride electrolytes because chlorine would be formed in the electrolytic treatment of zinc chloride electrolytes and would entail a considerable hazard.
Such a direct regeneration of a zinc chloride solution is disclosed in U.S. Pat. No. 4,073,709, according to which the solution containing chloride ions is introduced into a cathode chamber in an electrolysis cell which is divided into three chambers, namely an anode chamber, a cathode chamber, and an electrolyte chamber arranged therebetween. The anode chamber is defined by a porous membrane of low permeability which separates the anolyte from the electrolyte, the anolyte containing sulfuric acid. The anolyte contains a substance which is capable of binding to the chloride ions that enter the anode chamber and thus prevent oxidation of chloride ions at the anode. The liquid level of the anolyte is always maintained, by adding anolyte if necessary, so that the level is above the liquid level of the adjacent electrolyte for the purpose of sustaining the desired rate of flow through the membrane to achieve the technical purpose. In order to prevent any chloride ions that might seep through the anode membrane from being oxidized to chlorine gas, the anolyte contains a silver sulfate additive so as to assure the precipitation of the chloride as silver chloride.
The relatively complicated division of the electrolyte chamber into three chambers has been found problematical, as well as the use of membranes whose permeability can vary greatly in the course of the electrolytic process. Other problems can be seen in the addition of the silver sulfate chemical, in the formation of silver chloride and its removal from the cell, and in the danger of membrane clogging by silver chloride precipitates.
Furthermore, in the book, "Angewandte Elektrochemie," by A . Schmidt, Verlag Chemie Weinheim 1976, on page 210, requirements are given according to which zinc, in spite of its electronegative standard potential of -0.763 V, can be precipitated owing to the high overtension of the hydrogen on the zinc; it is stated that for the precipitation of zinc a relatively high zinc ion concentration is necessary at the cathode, since otherwise, due to the increasing sulfuric acid concentration, after a certain time hydrogen would separate instead of zinc. On page 213 of the same book various examples of zinc electrolysis methods are given.
EP 0 435 382 discloses an electrolysis process for treating old etchants containing metal ions. The cathode and anode chambers are separated from one another by an anion exchanger membrane, and the anode chamber is filled with a demetallized oxidizable or nonoxidizable etching solution. The freely chosen potential of the cathode or anode is kept constant by means of a voltage-regulated rectifier through a reference electrode; the metal ions are precipitated at the cathode and the regenerated acid concentrated in the anode chamber is returned to the etching bath.
However, no information can be found in EP 0 435 382 on the treatment of a solution containing metal ions with a sulfuric acid concentration that ranges from 60 to 80 grams per liter for an etching solution in need of regeneration.